Catalysts for purifying vehicle exhaust gas and the like are composed of a catalytic metal such as platinum, palladium, or rhodium, and a co-catalyst for enhancing the catalyst action of such metal, both supported on a catalyst support made of, for example, alumina or cordierite. The co-catalyst material absorbs oxygen under the oxidizing atmosphere and desorbs oxygen under the reducing atmosphere, and functions to optimally maintain the fuel/air ratio for efficient purification of noxious components in exhaust gases, such as hydrocarbons, carbon monoxide, and nitrogen oxides.
Efficiency of a catalyst for purifying exhaust gas is generally proportional to the contact area between the active species of the catalytic metal and exhaust gas. It is also important to maintain the fuel/air ratio at optimum, for which the reducibility associated with oxygen absorbing and desorbing capability of the co-catalyst should be maintained at a high level. However, a co-catalyst, such as cerium-containing oxides, is apt to be sintered during use at high temperatures, e.g., for exhaust gas purification. This results in reduction of its specific surface area, causing aggregation of the catalytic metals and decrease in the contact area between exhaust gas and the catalytic metals, which leads to reduction of efficiency in purifying exhaust gases.
In the light of the above, for improving the heat resistance of cerium oxide, Patent Publication 1 discloses methods of producing a ceric composite oxide containing silicon or the like elements, wherein ceric oxide is intimately mixed with an oxide of a metallic element such as silicon and calcinated; wherein ceric oxide is impregnated with an aqueous solution of a metal salt, such as silicate, which may be converted to an oxide by heating, and calcined; or wherein a precursor of a metal oxide, such as silicon oxide, is introduced into an aqueous colloidal dispersion of a cerium (IV) compound, a basic material is added to the dispersion to obtain a precipitate, the precipitate thus formed is subjected to solid-liquid separation and heat-treated. This publication also discloses that the amount of the oxide of a metallic element such as silicon is 1 to 20 mass %, preferably 1 to 5 mass % of the ceric oxide.
However, the ceric oxides containing 2.5 mass % SiO2 specifically produced in Examples 1, 5, and 6 of Patent Publication 1 exhibit specific surface areas of 20 m2/g at most as measured by the BET method after calcination at 900° C. for 6 hours. Further improvement is demanded.
For further improvement of the heat resistance of a cerium composite oxide containing silicon or the like as disclosed in Patent Publication 1, Patent Publication 2 discloses a process for the preparation of a composite oxide including the steps of suspending a ceric hydroxide having the formula Ce (M)x(OH)y(NO3)z, in which M is an alkali metal or a quaternary ammonium radical, x ranges from 0.01 to 0.2, y is such that y=4−z+x, and z ranges from 0.4 to 0.7, in an aqueous solution containing a decomposable base, such as ammonia, and a silicon compound, thermally treating the resulting suspension in a sealed container at less than the critical temperature and under less than the critical pressure thereof to form a medium of reaction, cooling the medium of reaction and releasing the medium of reaction to atmospheric pressure, separating ceric hydroxide therefrom, and calcining the ceric oxide thus separated, to thereby give a composite oxide wherein a silicon values is present in an amount of less than 2% by mass of the cerium values, expressed as SiO2.
In the Examples of Patent Publication 2, as shown in Table 1, a composite oxide having excellent heat resistance is disclosed, of which SiO2 value is in an amount of 0.94% by weight of the ceric oxide value, and which exhibits a specific surface area of 52 m2/g as measured by the BET method after calcination at 1000° C. for 6 hours.
However, Patent Publication 2 is silent about the reducibility of the obtained composite oxide, and the composite oxide obtained by the production method taught in this publication cannot achieve a sufficient reducibility.